1. Field of the Invention
The present invention relates to a method and an apparatus for controlling data transfer rates of data sources by guaranteeing that a marking threshold is a good estimate of the fair share of the available bandwidth to which data transfer rates are supposed to converge, in order to provide available bit rate service in asynchronous transfer mode networks.
2. Description of Related Art
A service category, called available bit rate (ABR), has recently been introduced in asynchronous transfer mode (ATM) networks to support applications that cannot be addressed efficiently by existing constant bit rate (CBR), variable bit rate (VBR), and unspecified bit rate (UBR) services. The introduction of ABR services has been motivated by the need for supporting traffic generated by highly-bursty data applications, where data packets are segmented into ATM cells. Most of these applications cannot predict their own traffic parameters, but have well-defined cell loss requirements, in order to avoid throughput collapse due to packet retransmission For these applications, a static allocation of resources at call setup time, as is used in CBR and VBR, would either be wasteful or inadequate. In ABR, only lower and upper bounds on the bandwidth of a connection are specified at call setup, and the ABR service guarantees a cell loss rate only to those connections whose source dynamically adapts its traffic in accordance with feedback received from the network. One issue with this service is to ensure fairness among the connections
The definition of the congestion control scheme used to generate feedback information from the network to dynamically regulate the cell generation process of each virtual connection (VC) relies on the use of special control cells called resource management (RM) cells, which are used to convey control information to the end points. Each data source regularly sends forward RM cells every NRM (where NRM is the number of data cells corresponding to the RM cell, e.g. 32) data cells. When the destination receives forward RM cells, it returns the RM cell to the data source as a backward RM cell. The switches in the network can also directly generate backward RM cells to communicate congestion to the source. The RM cell contains data transfer information that instructs the source to either increase or decrease its rate. In particular, RM cells contain a bit called a congestion indication (CI) bit, which is used to have a source increase or decrease its rate by some predefined amount, and a field, called the ER field, to carry a 16-bit floating-point number representing the explicit rate, which is an upper bound on the rate that a connection is allowed to transmit. The RM cells also contain a field, called the current cell rate (CCR) field, which is set by the source to its current allowed cell rate (ACR) when it generates a forward RM cell.
Each switch in the network that receives ATM traffic, controls congestion by implementing at least one of three methods: 1) EFCI marking, where the switch only marks a control bit in the data cells to indicate congestion, and relies on the destinations to convey information back to the sources by properly marking RM cells; 2) relative rate (RR) marking, where the switch uses RM cells to provide binary feedback on its state of congestion to each source; or 3) explicit rate (ER) marking, where the switch uses RM cells to explicitly provide the allowed rate to each source.
Many methods that can be implemented at the switches to control the data rates of the connections have been devised. Such existing methods, however, either do not perform satisfactorily, or have high implementation complexity Thus, a method and apparatus that achieves good performance by guaranteeing fairness and control on the buffer size and is simple to implement is desired.